Glass melting processes of the type disclosed in U.S. Pat. No. 4,519,814 (Demarest) involve heat recovery by directing exhaust gas from a melting zone into a batch material preheating zone. A duct carrying high temperature exhaust between the two zones is prone to having particulate and evaporated material carried with the exhaust deposited on its interior surfaces. This deposition of material can grow to the extent of adversely affecting the free flow of exhaust gas through the duct, thereby requiring periodic cleaning of the duct.
Opening the duct to scrape or blow the deposits off is undesirable because it is disruptive to the continuity of combustion conditions, exhaust gas flow patterns, and temperatures, which in turn can upset the melting process. Additionally, dislodging the deposited material and permitting it to enter the melting process can cause undesirable compositional variations in the product material. This is particularly a problem for glassmaking because even slight inhomogeneities in the glass result in variations in the refractive index that are perceived as distortion in the product glass. A compositional shift can also cause bubbles in the glass. The deposits differ in composition from the main batch material mixture because it is the more finely divided or volatile constituents of the batch mixture that become entrained disproportionately in the exhaust gas stream. Furthermore, manual removal of deposits is not easy and is sometimes required every few hours, thereby increasing the demands placed on operating personnel.
U.S. Pat. No. 4,678,491 (Tsai) discloses a technique for preventing deposition of material within an exhaust gas duct in a glass melting operation of the same type involved here. The technique involves directing a stream of air into the duct to alter the exhaust gas flow pattern within the duct to thereby prevent deposition of entrained material on selected areas of the duct. This approach may have merit with a relatively small operation where only small, specific areas are subject to accumulation of the deposits, but in a large system that approach may become impractical because of the large number of air streams needed to prevent deposition on the more extensive duct area involved. Injecting a large amount of additional gas into the duct is undesirable because it dilutes the exhaust gas, thereby reducing the exhaust gas temperature and reducing the efficiency of heat transfer from the exhaust gas to the batch materials in the preheating step. It is also undesirable to add to the volume of the exhaust gas stream because larger gas volume results in either a requirement for a larger preheating vessel or increased gas velocity with an associated greater entrainment of batch material in the preheating stage.
The problem is compounded in cases where the duct is cooled, such as by circulation of water through conduits in contact with the duct, for the sake of extending the service life of the duct. The relatively low temperatures of the interior surfaces of a cooled duct promote greater deposition and solidification of material over a wider area of the duct. Therefore, removal of deposits is required more frequently, and previous techniques have been found inadequate.